Removal of group VIII metal catalyst from polymer cements by extraction with aqueous dicarboxylic acid

ABSTRACT

A process is provided for removing Group VIII metal hydrogenation catalyst residue from a polymer which comprises oxidizing the hydrogenation catalyst residue in the polymer, contacting the polymer with an aqueous solution of dicarboxylic acid and separating the hydrogenation catalyst residue from the polymer. Preferred dicarboxylic acids include adipic and azelaic acids. The amount of water contacted with the polymer cement is less than that which would form an aqueous phase. Solid particulates comprising hydrogenation catalyst metal form within minutes upon the addition of the aqueous dicarboxylic acid to the polymer cement.

FIELD OF THE INVENTION

This invention relates to a process to prepare hydrogenated polymers.More particularly, the invention relates to the removal of residues ofhydrogenation catalysts from solutions comprising hydrogenated polymers.

BACKGROUND OF THE INVENTION

The uses of polymeric materials, including diolefin polymers, continuesto grow rapidly in such diverse areas as protective paint coverings,wire insulations, structural panels for automobiles, piping andlubricating oil viscosity index improvers. In many of theseapplications, the stability of the polymer is of paramount importance.Hydrogenation of diolefin polymers greatly improves the stability ofthese polymers against oxidative, thermal, and ultra violet degradation.Polymer hydrogenation processes have therefore been studied for manyyears as a method to prepare novel materials with excellent stabilityand other desirable properties. Early processes utilized heterogeneouscatalysts which were known to be useful for hydrogenation of lowmolecular weight olefins and aromatics. These catalysts includedcatalysts such as nickel on kieselguhr. A fine catalyst powder waspreferred and large amounts of catalysts were required to complete thehydrogenation in a reasonable amount of time. Such processes were onlypartially successful, since the reaction requires the diffusion of thepolymer molecules into the pores of the catalyst, where the activenickel metal was present. This is a slow process when hydrogenatingpolymers.

Discovery of nickel octoate/triethyl aluminum hydrogenation catalystsystems enabled rapid hydrogenation of polymers. These processes utilizethe catalyst as a colloidal suspension in polymer containing solutions.This type of catalyst is referred to as a homogeneous catalyst. Such aprocess has been used for a number of years to prepare hydrogenatedbutadiene-styrene polymers. U.S. Pat. No. 3,554,991 describes anexemplary process. Besides nickel, Group VIII metals in general willfunction as the active metal in these systems, and in particular, iron,cobalt, and palladium are known to be acceptable.

Pore diffusion is not a limitation with homogeneous catalysts and thehydrogenation process is rapid and complete in a matter of minutes.However, removal of the catalyst from the polymer product is necessarybecause metals, and particularly nickel, which remain with the polymercatalyze degradation of the polymer product. The removal of the catalystfrom polymer solutions is commonly accomplished by the addition of anammonium phosphate-water solution and air and then filtration of solidswhich contain the catalyst particals from the polymer solution. The airis utilized to oxidize the nickel to a divalent state.

Alternative methods to remove hydrogenation catalyst residues fromcements of hydrogenated polymers include treatment with an aminecompound wherein the amine is either a chloride salt or a diamine havingan alkyl group of 1 to 12 carbon atoms as disclosed by U.S. Pat. No.4,098,991; and treatment with a non-aqueous acid followed byneutralization with an anhydrous base and filtration, as disclosed byU.S. Pat. No. 4,028,485.

Some of these catalyst removal systems are undesirable because thoseprocesses require relatively expensive metallurgy due to the corrosivenature of the nickel removal compounds. Many also produce an aqueousacidic sludge containing the catalyst and residues of the treatmentchemicals. It can be difficult and expensive to dispose of this sludge.

Treatment of polymer cements to remove hydrogenation catalyst residuescan also be accomplished by contacting the cement with dicarboxylic acidand an oxidant, as disclosed in U.S. Pat. No. 4,595,749. In thisprocess, the dicarboxylic acid is first dissolved in toluene, ethanol,or another solvent for the polymer. This method is advantageous becauseit can be accomplished in equipment fabricated from inexpensivematerials. This process also does not produce an acidic aqueous streamwhich requires disposal because the catalyst residue metals and acidprecipitate directly from the polymer cement. However, it has been foundthat this process has the disadvantage of requiring an excessive amountof time for the precipitate to form.

It is therefore an object of this invention to provide a process toremove Group VIII metal containing hydrogenation catalyst residue frompolymer cements. It is a further object of this invention to provide aprocess to remove hydrogenation catalyst residue from polymer cementswhich does not produce an aqueous phase of reactants. It is anotherobject to provide a process where hydrogenation catalyst residue can beremoved from polymer cements by precipitation of the residues whereinthe precipitation is rapid and results in solid particles which areeasily removed from the polymer cements. It is another object of thisinvention to provide a process which is capable of removinghydrogenation catalyst residues from polymer cements to a level of 10ppm of Group VIII metals or less based on the polymer.

SUMMARY OF THE INVENTION

The objects of this invention are accomplished by a process comprisingthe steps of providing a hydrogenation catalyst residue containingpolymer cement, oxidizing the hydrogenation catalyst residue, contactingthe oxidized hydrogenation catalyst residue with an aqueous solution ofdicarboxylic acid and recovering a polymer cement comprising less than10 ppm by weight, based on the polymer, of Group VIII metals.

In a preferred embodiment, the oxidation is accomplished using aperoxide before the polymer cement is contacted with the dicarboxylicacid, and the polymer cement is recovered by filtration from theresulting precipitated hydrogenation catalyst metal particles. Preferreddicarboxylic acids include adipic and azelaic acids. The amount of watercontacted with the polymer cement is less than that which would form aseparate aqueous phase which eliminates the need to separate two liquidphases. Solid particulates comprising hydrogenation catalyst metal whichare readily seperated from the cement form within minutes upon theaddition of the aqueous dicarboxylic acid to the polymer cement.

DETAILED DESCRIPTION OF THE INVENTION

The polymer cements of the present invention preferably comprise from 1to about 30 parts by weight of a polymer, and more preferably comprisefrom about 10 to about 12 parts by weight of polymer, based on the totalamount of cement, in an inert hydrocarbon solvent. The hydrocarbonsolvent may be an aliphatic solvent or an aromatic solvent and typicallycomprises hydrocarbons having from 4 to 20 carbon atoms. The polymer isa partially, selectively, or totally hydrogenated polymer. The presentinvention does not depend upon the type of nature of the polymer. Thepolymer may therefore be a thermoplastic polymer, or an elastomericpolymer and may have a molecular weight which varies between widelimits. Most typically, polymers which are benefited by hydrogenationare those comprising polymerized conjugated diolefins. These conjugateddiolefin containing polymers are therefore preferred for the practice ofthe present invention. They may be prepared by radical, anionic orcationic polymerization and may be copolymers with other monomer units.The copolymers may be random, block, or tapered, and may have structuresthat are linear, branched or radial.

In a most preferred embodiment, the polymer is an anionicallypolymerized conjugated diolefin polymer which was polymerized in aninert solvent, and then hydrogenated in the same solvent to form thehydrogenation catalyst residue containing polymer cement.

When an anionic initiator is used, polymers may be prepared bycontacting the monomers with an organoalkali metal compound in asuitable solvent at a temperature within the range from about -100° C.to about 300° C., preferably at a temperature within the range fromabout 0° C. to about 100° C. Particularly effective polymerizationinitiators are organolithium compounds having the general formula:

    RLi.sub.n

Wherein R is an aliphatic, cycloaliphatic or aromatic hydrocarbonradical having from one to about 20 carbon atoms; and n is an integer of1 to 4.

When the polymer is a block copolymer, the copolymer is preferably astyrene-conjugated diolefin block copolymer. This is due to thethermoplastic and elastomeric nature of these polymers. The polystyrene,being incompatible with the polyconjugated olefins, form separatedomains, and these domains have relatively high glass transitiontemperatures. Above the glass transition temperatures of the polystyrenedomains the polymer is in a melt phase and can be molded, extruded orblended with other components. Below the glass transition temperature ofthe polystyrene the polystyrene domains are hard and act as physicalcrosslinks between the rubbery conjugated diolefin chains. This resultsin excellent elastomer properties along with reprocessability.

The polymer of the present invention is typically contacted withhydrogenation catalyst and hydrogen in an inert solution with a solventsuch as cyclohexane, normal hexane, pentane, heptane or octane. When thehydrogenation conditions are not sever enough to saturate aromatics,solvents such as toluene, xylene and benzene may be used.

The hydrogenation catalysts themselves have complex structures which arenot well understood and are therefore usually described by the processused to prepare them. The hydrogenation catalyst can be prepared bycombining a Group VIII metal carboxylate or alkoxide ("catalyst") withan alkyl or hydride of a metal selected from Groups I-A, II-A and III-Bof Medeleev's Periodic Table of Elements ("cocatalyst"). The preparationof such catalysts is taught in U.S. Pat. Nos. 3,591,064 and 4,028,485,which are incorporated herein by reference. For effective hydrogenationunder reasonable hydrogen pressures, temperatures and times,concentrations of from about 1×10⁻³ to about 20×10⁻³ mmoles of GroupVIII metal per gram of polymer and cocatalyst to catalyst ratios ofbetween about 1 and about 4 are acceptable.

The catalyst metals which are preferred include iron, cobalt, nickel andpalladium because of the availability of ample data on the performanceof these metals in hydrogenation catalysts. Nickel and cobalt are mostpreferred. Iron is not most preferred because it is less active than theothers and palladium is not most preferred because it is more expensivethan nickel and cobalt.

Hydrides or alkyls of lithium, magnesium or aluminum are preferredcocatalysts due to the excellent hydrogenation activity of the systemscontaining these cocatalysts.

The hydrogenation catalysts are insoluble in the polymer cements andform a colloidal suspension. They are typically although impreciselyreferred to as "homogeneous" catalyst systems to differentiate them fromhydrogenation catalysts which are porous solids.

Hydrogenation catalyst residues in polymer cements comprise Group VIIImetal ions in a state of zero valence. This is indicated by a blackcolor imparted to the polymer cement by nickel hydrogenation catalystresidue. Upon oxidation, the black color will turn green, indicationthat the valence of the nickel has become plus two. Oxidizing thesemetal ions enhances the agglomeration and precipitation of the metalparticles by dicarboxylic acid because the dicarboxylic acid racts veryslowly with the metal ions which have a valence of zero.

The hydrogenation catalyst residue metals may be oxidized by any knownmethod of oxidizing these metals. It is preferred that they be oxidizedby contacting the polymer cement with an oxidizer such as molecularoxygen or a peroxide. The peroxide may be hydrogen peroxide or anorganic peroxide.

When oxygen is used to oxidize the catalyst metals, amounts may rangefrom 0.1 to 100 moles of oxygen per mole of metal, but a molar ratiobetween 0.1 and 5 is preferred. Hydrogen peroxide requirements may rangefrom 0.1 to 100 moles per mole of metal in the hydrogenation product,with 0.1 to 5 the preferred range of the molar ratio. The quantities ofalkyl hydroperoxides needed range from 0.1 to 100 moles per mole ofmetal, but preferably a molar ratio of 0.1 to 5 should be used.

Hydrogen peroxide may be added as an aqueous solution of from 1 to about40 percent hydrogen peroxide in water. Organic peroxides such as alkylor aryl hydroperoxides suitable for this invention may be primary,secondary or tertiary alkyl hydroperoxides, although the tertiary alkylhydroperoxides are preferred. Examples are ethyl peroxide, butylhydroperoxides, isopropyl hydroperoxide, tertiary butyl hydroperoxideand the like. Tertiaryl butyl hydroperoxide is a preferred oxidant.

The product may be contacted with oxidant before, or simultaneously withtreatment with the acid. The oxidant may be bubbled through the polymercement, or it may be added as a liquid or solution, as with hydrogenperoxide or tertiary alkyl hydroperoxide. When oxidant is bubbled orsparged through the polymer cement the oxidant may be a gas streamcomprising a major portion of nitrogen and a minor portion of oxygen,such as air.

The dicarboxylic acid can be any organic acid containing two or morecarboxylic acid groups. It has been theorized that dicarboxylic acidsremove nickel and other metal ions from polymer cements by formingpolymeric chains with the metal ions linking carboxylic acid groups fromtwo different dicarboxylic acid molecules. The dicarboxylic acidpreferably comprises from about 2 to about 12 carbon atoms andpreferably has carboxylic acid functionality at terminals of carbon atomchains.

The only significant limitation on the dicarboxylic acids which may beutilized is that the dicarboxylic acids must be relatively soluble inwater. The dicarboxylic acid must be soluable to the extent of 1 gram ormore per 100 grams of water and more preferably 5 grams or more per 100grams of water. The water may be heated to enable dissolution of thisamount of dicarboxylic acid and is preferably heated to about 50° C. orgreater to ensure dissolution of the dicarboxylic acid.

Acceptable dicarboxylic acids include, but are not limited to, succinic,oxalic, tartaric, malonic, fumaric, maleic, sebacic, adipic, azelaic andphthalic acids.

Disolving the dicarboxylic acid in water before contacting the cementwith the dicarboxylic acid has been found to be critical for rapidformation of hydrogenation catalyst residue precipitates. Withoutpredissolving the dicarboxylic acid in water particulates which can beremoved from the polymer cement form, but they form at a rate which istoo slow for commercial application of the process.

The amount of dicarboxylic acid which is utilized is preferably morethan stoichiometric to the metal ions to be removed from the polymercement. An amount between about 100% and about 200% of stoichiometric ismost prefered. Excess dicarboxylic acid needlessly contaminates thepolymer cement, and an lower amount of dicarboxylic acid results ininsufficient metal ion removal.

The treated polymer cement may be recovered from the polymer cementmixture by any known means to separate solids from viscous liquids.Centrifugal means such as centrifuges or cyclones may be utilized.Filtering, preferably in the presence of a filter aid, may also beutilized, along with gravity settlement such as decantation or parallelplate separators. Filtering in the presence of a filter aid is preferredbecause this method is known to be effective to separate fine particlesfrom polymer cements.

The use of aqueous solutions of dicarboxylic acids results in muchfaster precipitation of hydrogenation catalyst residues than the use ofsolutions of dicarboxylic acids in toluene or ethanol as taught by U.S.Pat. No. 4,595,749. The examples in '749 demonstrated acceptable gravitysettlement of particulates, but the samples stood over night for theseparation to become evident. With the present invention, theparticulates form and solid phase precipitation of the catalyst residuecontaining particles from the polymer cement is evident within aboutfive minutes.

EXAMPLES

Two hydrogenation catalyst residue containing polymer cements wereprepared by anionically polymerizing a styrene-butadiene block copolymerin cyclohexane and then hydrogenating the polymer in the same solution.The cements varied in that one cement contained 12 percent by weightpolymer and the other contained 16 percent by weight polymer. A butyllithium initiator was utilized to anionically polymerize the polymers.The polymer was a triblock copolymer having polystyrene end blocks ofnumber average molecular weights of about 7500 each. The midblock waspolybutadiene with a number average molecular weight of about 37,000.The polymerization was terminated by adding a small amount of methanol.

The polymer solutions were then hydrogenated by adding nickel(2-ethylhexanoate) and triethylaluminum. The solution was then held atabout 80° C. under about 700 psia hydrogen partial pressure for aboutthree hours to accomplish hydrogenation of the polymer. These cementswere then divided and treated as described below.

Hydrogen peroxide as either a 3 or a 30 percent by weight solution inwater was used to oxidize the catalyst metals. Sufficient hydrogenperoxide solution was added to result in a molar ratio of nickel toperoxide of about 1:4. The cements turned from black to green withinabout five minutes.

Adipic acid was added to the oxidized polymer cement as a as a 5 percentby weight solution in 50° C. deionized water. Azalaic acid was utilizedas a 5 percent by weight solution in 75° C. deionized water. Ascomparative examples, the acids were added to the polymer cements aspowders. In each sample, the acid was added in about a 25% excess ofstoichiometric to the nickel. After the acids were added, the solidswere allowed to settle. Table 1 lists the cements used, the acids used,the temperatures of the cements at the time the acids were added andduring the precipitation periods, the hydrogen peroxide concentrationsused, the cement nickel and aluminum concentrations before and after thetreatment, and observations of the rate and form of the precipitate.

                                      TABLE 1                                     __________________________________________________________________________    Polymer           H2O2                                                                              Sol Feed - Product                                                                        Precipitate                                 in Cement  Acid   Conc.                                                                             Ni Al Ni Al Rate                                                                             Form                                     Sample                                                                            % wt                                                                              °C.                                                                       type                                                                              form                                                                             % wt                                                                              (ppm-based on the polymer cement)                       __________________________________________________________________________    1   12  25 adipic                                                                            sol.                                                                             3   150                                                                              166                                                                              0.58                                                                             4.35                                                                             Fast                                                                             Fluffy                                   2   12  60 adipic                                                                            sol.                                                                             3   150                                                                              166                                                                              0. 0. Fast                                                                             Fluffy                                   3   16  60 adipic                                                                            sol.                                                                             3   225                                                                              183                                                                              0. 0. Fast                                                                             Fluffy                                   4   12  70 adipic                                                                            pow.                                                                             3   150                                                                              166                                                                              0.05                                                                             0. Slow                                                                             Dense                                    5   16  25 adipic                                                                            sol.                                                                             3   225                                                                              183                                                                              0.03                                                                             1. Fast                                                                             Fluffy                                   6   12  70 adipic                                                                            pow.                                                                             30  150                                                                              166                                                                              0. 3.85                                                                             Slow                                                                             Dense                                    7   12  25 adipic                                                                            sol.                                                                             3   225                                                                              183                                                                              0.08                                                                             4.48                                                                             Fast                                                                             Fluffy                                   8   12  25 azel.                                                                             sol.                                                                             3   150                                                                              166                                                                              0.10                                                                             5.27                                                                             Fast                                                                             Fluffy                                   9   12  25 adipic                                                                            sol.                                                                             30  150                                                                              166                                                                              0. 0. Fast                                                                             Fluffy                                   10  16  25 adipic                                                                            pow.                                                                             3   225                                                                              183                                                                              0.73                                                                             0.98                                                                             Slow                                                                             Dense                                    11  16  70 azel.                                                                             pow.                                                                             3   225                                                                              183                                                                              0. 3.35                                                                             Slow                                                                             Dense                                    12  12  25 adipic                                                                            sol.                                                                             3   150                                                                              166                                                                              0. 0. Fast                                                                             Fluffy                                   __________________________________________________________________________

From Table 1 it can be seen that when the acid is added to the polymercement in the form of a powder the precipitate of the catalyst metalsforms slowly and forms as a dense precipitate. The formation of theprecipitate is independent of the concentration of the peroxide addedand the temperature of the cement when the acid is added. All twelvesamples resulted in useful treated cements in that each had less that 10ppm (on polymer basis) of Group VIII metal.

I claim:
 1. A process to remove Group VIII metal containinghydrogenation catalyst residue from a polymer which has beencatalytically hydrogenated, the process comprising the steps of:(a)oxidizing the hydrogenation catalyst residue in the polymer; (b)contacting the polymer with an aqueous solution of a dicarboxylic acid;(c) allowing a hydrogenation catalyst residue precipitate to form; and(d) separating the hydrogenation catalyst residue from the polymer. 2.The process of claim 1 wherein the hydrogenation catalyst was preparedby combining a Group VIII metal carboxylate or alkoxide with an alkyl orhydride of a metal selected from Groups I-A, II-A and III-B ofMedeleev's Periodic Table of Elements.
 3. The process of claim 1 whereinthe hydrogenation catalyst is prepared by combining a component selectedfrom the group comsisting of nickel carboxylate, nickel alkoxide, cobaltcarboxylate, iron alkoxide, palladium carboxylate and palladium alkoxidewith a component selected from the group consisting of lithium alkyl,lithium hydrate, magnesium alkyl, magnesium hydrate, aluminum alkyl andaluminum hydrate.
 4. The process of claim 1 wherein the hydrogenationcatalyst is prepared by combining nickel (2-ethylhexanoate) and analuminum alkyl.
 5. The process of claim 4 wherein the aluminum alkyl istriethylaluminum.
 6. The process of claim 1 wherein the dicarboxylicacid is selected from the group consisting of adipic acid and azelaicacid.
 7. The process of claim 1 wherein the hydrogenation catalystresidue is in the form of a colloidal suspension.
 8. The process ofclaim 1 wherein the polymer is in a solution of from about 1 to about 30percent by weight polymer based on the total polymer solution, in aninert solvent.
 9. The process of claim 8 wherein the inert solvent isselected from the group comprising cyclohexane, toluene, hexane, andbenzene.
 10. The process of claim 1 wherein the polymer is a polymerwhich comprised conjugated diolefin monomer units prior to beinghydrogenated.
 11. The process of claim 10 wherein the polymer was,before hydrogenation, a copolymer comprising conjugated diolefin monomerunits and styrene monomer units.
 12. The process of claim 11 wherein thecopolymer was a block copolymer comprising at least one block whichcomprises monomer units of conjugated diolefins and at least one blockwhich comprises styrene monomer units.
 13. The process of claim 8wherein the polymer is anionically polymerized in the inert solventprior to hydrogenation of the polymer.
 14. The process of claim 1oxidation is accomplished by contacting the polymer with hydrogenperoxide.
 15. The process of claim 1 wherein the oxidization isperformed by contacting the polymer with oxygen.
 16. The process ofclaim 15 wherein the oxygen is contacted with the hydrogenation catalystresidue containing polymer cement by bubbling a gas stream comprisingoxygen through the polymer cement.
 17. The process of claim 16 whereinthe gas stream comprises a major portion of nitrogen and a minor portionof oxygen.
 18. The process of claim 14 wherein the oxidation isaccomplished by contacting the polymer with an alkyl hydroperoxide. 19.The process of claim 1 wherein the oxidization is accomplished when thepolymer is contacting the dicarboxylic acid.
 20. The process of claim 19wherein the oxidization is accomplished by contacting the polymer with aperoxide.
 21. The process of claim 1 wherein the polymer is separatedfrom the hydrogenation catalyst residue by filtration.
 22. The processof claim 21 wherein the filtration is performed utilizing a filter aid.23. The process of claim 1 wherein the polymer is separated from thehydrogenation catalyst residue by gravity settlement.
 24. The process ofclaim 1 wherein the polymer is separated from the hydrogenation catalystresidue by centrifugation.